This invention relates to an electron gun for a cathode ray tube. More specifically, this invention relates to a technique to improve high frequency magnetic field transmission property of an electron gun.
FIG. 5 shows a structure of a conventional electron gun for a projection-type monochrome cathode ray tube disclosed in Unexamined Published Japanese Patent Application (Tokkai-Hei) 10-74465. In FIG. 5, 14 denotes a neck tube having an electron gun disposed inside the tube. The electron gun is formed by sequentially arranging a cup-shaped G1 electrode (control electrode) 5 housing a cathode 6, a cup-shaped G2 electrode (acceleration electrode) 7, a tubular G3 electrode (pre-anodic electrode) 8, a G4 electrode (focusing electrode) 9, and a G5 electrode (anodic electrode) 10 enveloping the top end part of the G4 electrode 9. A main electron lens is formed between the G3 electrode 8 and the G4 electrode 9. Another electron lens is formed inside the G5 electrode 10 at a position between the G4 electrode 9 and the G5 electrode 10. Outside of the neck tube 14, a velocity modulation coil 18, a convergence yoke 15, and a deflection yoke 16 are disposed.
As shown in FIG. 5, the state-of-the-art for improving focusing performance is subjecting the electron gun disposed inside the neck tube 14 to magnetic field modulation caused by the velocity modulation coil 18 from outside of the neck tube 14 in order to carry out velocity modulation of an electron beam. Namely, a track of an electron beam outgoing from a cathode 6 is modulated by an alternating magnetic field generated by the deflection yoke 16, the convergence yoke 15, the velocity modulation coil 18 and the like, before the electron beam reaches a phosphorous screen surface. The deflection yoke 16, which is attached to a funnel cone portion of the cathode ray tube, generates an alternating magnetic field 17 to deflect an electron beam track, so that the electron beam scans the phosphorous screen surface of the cathode ray tube. The convergence yoke 15, attached to the outside of the neck tube 14 of the cathode ray tube, corrects raster distortion and color displacement by generating an alternating magnetic field 20 to modulate the electron beam track. The velocity modulation coil 18 is attached to the outside of the neck tube 14 of the cathode ray tube and generates alternating magnetic field 19 to modulate the scanning speed of the electron beam in order to prevent a high-intensity part on the phosphorous screen from extending to a low-intensity part and to sharpen images.
The frequency of an alternating magnetic field for modulating an electron beam reaches a mega-Hertz order equivalent to a frequency for images. Therefore, when an electron gun includes metal portions formed by deep-drawing metal materials such as stainless steel, the alternating magnetic field is damped and a desired electrode beam modulation cannot be obtained.
As shown in FIG. 5, most of the alternating magnetic field 20 generated by the convergence yoke 15 passes the G5 electrode 10. The deflection yoke 16 is attached to the funnel cone portion. A portion of the alternating magnetic field 17 generated by the deflection yoke 16 passes the G5 electrode 10. The velocity modulation coil 18 is disposed between the G3 electrode 8 and the G4 electrode 9. Most of the alternating magnetic field 19 generated by the velocity modulation coil 18 passes the G3 electrode 8 and the G4 electrode 9.
When an alternating magnetic field is applied through these metal electrodes, eddy current is generated at the metal electrode. The eddy current loss is increased as the frequency of the alternating magnetic field becomes high. Thus, modulation effect of the electron beam track due to the magnetic field in the high frequency modulation band is reduced. For example, eddy current is generated at the G5 electrode 10 due to the alternating magnetic field 20 generated by the convergence yoke 15. This decreases the electron beam track modulation effect provided by the convergence yoke 15.
Furthermore, this eddy current loss can heat the electrodes and break the neck tube. If the source of the alternating magnetic field and the metal electrodes of the electron gun are positioned fully apart in order to prevent the loss of the alternating magnetic field or the electrode heat, the electron beam focusing lens is arranged inevitably separated from the phosphorous screen surface. As a result, the electron beam magnification becomes high and the resolution is lowered. Especially for image display apparatuses having high deflection frequencies and wide signal bands such as high definition television, the loss in the alternating magnetic field is increased. This increased loss causes problems in use.
Tokkai-Hei 8-115684 discloses the improvement of transmission property of the magnetic field by dividing the deep-drawn metal portions into several sections and providing clearances between the respective sections. However, this method causes problems such as deterioration in assembly accuracy or increased cost. Moreover, in order to maintain the mechanical strength, the divided sections cannot be made too small and thus, the magnetic field transmission property cannot be improved remarkably.
It is an object of one or more embodiments of this invention to solve these problems and provide a cathode ray tube having an electron gun that can provide a desired electron beam modulation effect without interrupting transmission of the modulation magnetic field from the exterior of the vacuum portion.
An electron gun in accordance with an embodiment of the present invention includes a tubular electrode for an electron beam to pass through the inside and at least one part of the tubular portion of the electrode is formed into a coiled portion. Accordingly, a modulation magnetic field passes through the clearances between parts of the coiled portion and thus, eddy current loss can be decreased.
Preferably in one or more embodiments of the electron gun, at least one part of the pre-anodic electrode (G3 electrode) is formed into a coiled portion, so that an equipotential space can be formed inside the pre-anodic electrode.
Preferably, in one or more embodiments, the coiled portion is composed of a nonmetal material, so that the transmission effect of the modulation magnetic field is further improved.
In an embodiment the electron gun, the coiled portion may be a coiled wire.
Preferably, in one or more embodiments, the coiled portion is formed so that clearances between the parts adjacent to each other in the axial direction are not more than 2.5 mm. Accordingly, influences from the outer electric field can be reduced. Furthermore, the coiled portion may be formed so that the parts adjacent in the axial direction are contacted with each other. Accordingly, the strength of the electrode members can be improved while maintaining the effect of transmission of the modulation magnetic field.
A manufacturing method in accordance with an embodiment of the present invention is applied to provide an electron gun having a tubular electrode for passing an electron beam inside the electrode, in which at least one part of the tubular portion of the electrode is formed into a coiled portion. In the method, a coiled portion is formed by cutting spirally a tubular electrode member and then stretching the electrode member in the axial direction. According to the method, the coiled portion can easily be manufactured.
A cathode ray tube device in accordance with an embodiment of the present invention includes a cathode ray tube having an electron gun inside the neck portion, and a velocity modulation coil outside the cathode ray tube. At least one part of a pre-anodic electrode of the electron gun is formed into a coiled portion, and the velocity modulation coil is provided around the coiled portion of the pre-anodic electrode. Accordingly, velocity modulation effect can be improved.